Concurrent - Semaphore - acquire(int permits)

原创转载请注明出处：http://agilestyle.iteye.com/blog/2342898

有参方法acquire(int permits)表示每调用1次此方法，就使用Semaphore中的x个permits

Service.java

package org.fool.java.concurrent.semaphore.acquire;

import java.util.concurrent.Semaphore;

public class Service {
    private Semaphore semaphore = new Semaphore(10);

    public void testMethod() {
        try {
            semaphore.acquire(2);

            System.out.println(Thread.currentThread().getName() + " begin timer=" + System.currentTimeMillis());

            int sleepValue = 5000;
            System.out.println(Thread.currentThread().getName() + " stop " + (sleepValue / 1000) + " sec");
            Thread.sleep(sleepValue);

            System.out.println(Thread.currentThread().getName() + " end timer=" + System.currentTimeMillis());

            semaphore.release(2);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Note:

private Semaphore semaphore = new Semaphore(10);

定义10个permits，每次执行semaphore.acquire(2);消耗掉2个，所以10/2=5，说明同一时间只允许5个线程执行acquire()和release()之间的代码。 

ThreadA.java

package org.fool.java.concurrent.semaphore.acquire;

public class ThreadA implements Runnable {
    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        service.testMethod();
    }
}

SemaphoreTest.java

package org.fool.java.concurrent.semaphore.acquire;

public class SemaphoreTest {
    public static void main(String[] args) throws InterruptedException {
        Service service = new Service();

        for (int i = 0; i < 10; i++) {
            new Thread(new ThreadA(service)).start();
        }
    }
}

Run

 

如果多次调用Semaphore的release()或release(int)方法时，还可以动态增加permits的个数。

package org.fool.java.concurrent.semaphore.acquire;

import java.util.concurrent.Semaphore;

public class SemaphoreTest2 {
    public static void main(String[] args) {
        try {
            Semaphore semaphore = new Semaphore(5);

            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();
            semaphore.acquire();

            System.out.println(semaphore.availablePermits());   // outputs 0

            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();
            semaphore.release();

            System.out.println(semaphore.availablePermits());   // outputs 6

            semaphore.release(4);

            System.out.println(semaphore.availablePermits());   // outputs 10
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

Note:

Semaphore semaphore = new Semaphore(5);

 定义的5并不是最终的permits数量，仅仅是初始的状态值。